You can determine whether it is purpura by simple manifestation and pressure. However, it is necessary to consult a doctor in time in order to clarify the cause of purpura and make timely diagnosis and treatment. 1. Purpura is bleeding of the skin and mucous membranes with a diameter of 3~5mm. It is mainly formed by blood stagnation under the skin or mucous membranes, and is usually manifested as red or dark red plaques. 2. If the plaque does not fade when pressed, it can be judged as purpura. Different causes of purpura can have different manifestations. For example, allergic purpura is often manifested as symmetrical, elevated skin purpura on the limbs or arms, which may be accompanied by itching, joint pain, abdominal pain, etc. Simple purpura is often chronic occasional petechiae on the limbs. If there is any discomfort, it is recommended to consult a doctor in time, under the guidance of a physician.